The work herein was supported in part by a grant from the Robert A. Welch Foundation.
This invention relates to a strategy for determining the enantiomeric purity of a compound through guest-host complexation, spectroscopy, particularly UV absorption spectroscopy and fluorescence spectroscopy, and chemometric modeling.
The need for improved strategies for the assessment of enantiomeric purity arises from increased pressure on the pharmaceutical industry by government agencies for documentation on the pharmacological effects of individual enantiomers and the simultaneous demand in drug development for determination of enantiomeric excess in large combinatorial libraries. While many analytical techniques for chiral analysis have been developed over the years, gas and liquid chromatography, capillary electrophoresis and nuclear magnetic resonance are currently the most widely used. For high throughput screening strategies, slow chromatographic methods are not attractive. Rapid spectroscopic techniques are the most desirable.
Experimental discrimination of enantiomers is carried out conventionally by means of chiral auxiliary agents such as chiral shift reagents, chiral complexing agents, and chiral solvents. (Sullivan, G.R., Top. Stereochem., vol. 10, pp. 287–329, 1978). This diastereomeric discrimination arises when a given enantiomer of the chiral auxiliary interacts with two enantiomers of a compound to produce diastereomeric pairs with different physical properties as shown below in Scheme 1, where RCA is a chiral auxiliary.

Chromogenic enantioselective chiral hosts are capable of discriminating between enantiomers of chiral guests through a change in the visible absorption spectrum of the enantioselective complex, i.e., through a color change. (Otagiri, et al., Chem. Pharm. Bull., vol. 23, p. 188, 1975; Schiller, et al., J. Chem. Soc., Faraday Trans., vol. 83, p. 3227, 1987; Park, et al., J. Phys. Chem., vol. 98, p. 6158, 1994; Cox, et al., J. Photochem. Photobiol., vol. 39, p. 597, 1984; Bortolus, et al., J. Phys. Chem. A, vol. 106, p. 1686, 2002; Balabai, J. Phys. Chem., vol. 102, p. 9617, 1998). Under this strategy, the complexation of one enantiomer of a chiral substrate with a chiral host results in a visible spectral shift and/or the formation of an entirely new visible band, while little or no color change is observed when the other enantiomer complexes with the chiral host.
Traditonally, cyclodextrins are used as host molecules. Cyclodextrins (“CDs”) are homochiral barrel-shaped sugar molecules that can form transient, non-covalent diastereomeric guest-host complexes with chiral guest molecules. Because the complexes that are formed are diastereomeric, they have different physical properties. Consequently, there are small changes in their spectra. (Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules, p. 102, 1967). These small spectral variations are often dismissed as having little utility for predicting the composition of a sample because the variations are small, the bands overlap, and the spectra do not appear to show a consistent trend (such as an offset) with composition. However, chemometric methods, such as multivariate regression, offer a variety of powerful techniques for revealing hidden relationships in data that are not immediately apparent.
Multivariate regression modeling (“MRM”) is widely used in chemistry as a means of correlating spectral data with known compositional changes. (Martens, et al., Multivariate Calibration, 1989). While the use of chemometrics in near-infrared spectroscopy is well-established, its use in other spectral regions, such as the ultraviolet region, is not as common. MRM is used for the chemometric analysis of the spectral data of the solutions containing cyclodextrin guest-host inclusion complexes because the solution spectra are composite spectra, simultaneously containing contributions from complexed species (diastereomeric CD inclusion complexes) as well as uncomplexed species that are present because the complexation reaction is not complete.